Apparatus and process for sensing fluoro species in semiconductor processing systems

ABSTRACT

A gas detector and process for detecting a fluorine-containing species in a gas containing same, e.g., an effluent of a semiconductor processing tool undergoing etch cleaning with HF, NF 3 , etc. The detector in a preferred structural arrangement employs a microelectromechanical system (MEMS)-based device structure and/or a free-standing metal element that functions as a sensing component and optionally as a heat source when elevated temperature sensing is required. The free-standing metal element can be fabricated directly onto a standard chip carrier/device package so that the package becomes a platform of the detector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 10/273,036filed Oct. 17, 2002 for “APPARATUS AND PROCESS FOR SENSING FLUOROSPECIES IN SEMICONDUCTOR PROCESSING SYSTEMS” in the names of Frank DimeoJr., Philip S. H. Chen, Jeffrey W. Neuner, James Welch, Michele Stawasz,Thomas H. Baum, Mackenzie E. King, Ing-Shin Chen, and Jeffrey F. Roeder.

GOVERNMENT RIGHTS IN INVENTION

Work related to the invention hereof was conducted in the performance ofNIST ATP Program, Contract Number 70NANB9H3018. The Government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a sensor for fluoro speciesand to a method of sensing such species, having utility for monitoringof fluorine-containing compounds and ionic species in semiconductorprocess operations.

2. Description of the Related Art

In the manufacture of semiconductor devices, the deposition of silicon(Si) and silicon dioxide (SiO₂), and subsequent etching, are vitaloperational steps that currently comprise 8-10 steps or roughly 25% ofthe total manufacturing process. Each deposition tool and etch tool mustundergo a periodic cleaning procedure, sometimes as often as every run,in order to ensure uniform and consistent film properties.

Currently, in etching operations, etch endpoints are reached when aprescribed amount of time has elapsed. Over etch, in which the processgas continues to flow into the reactor chamber after the cleaning etchis finished, is common and leads to longer process cycles, reduced toollifetimes, and unnecessary global-warming-gas losses to the atmosphere(Anderson, B.; Behnke, J.; Berman, M.; Kobeissi, H.; Huling, B.; Langan,J.; Lynn, S-Y., Semiconductor International, October (1993)).

Similar issues are present in the etching of silicon nitride materialswhen SiN is utilized in semiconductor device structures.

Various analytical techniques, such as FTIR, Optical EmissionSpectroscopy, and Ionized Mass Spectroscopy, can be used to monitor theetch process. However, these techniques tend to be expensive, and oftenrequire a dedicated operator due to their complexity.

It would therefore be a significant advance in the art to provide areliable, low-cost gas sensing capability that will serve to improve thethroughput and chemical efficiency of the equipment used for thedeposition and etching of silicon-containing materials, includingsilicon, silicon nitride and silicon dioxide, by reducing and optimizingclean and etch times, and hence reducing chemical usage, lengtheningequipment operating life, and decreasing equipment down time.

In addressing this need, micromachined gas sensor devices wouldconceptually be useful to provide high performance sensing, due to theiramenability to fabrication of suspended structures that can bemanipulated thermally in a rapid manner. Surface micromachined deviceshave been developed using standard 2-level CMOS processing. In thefabrication of process sensors for aggressive environments, however, amajor problem is protection of the sensor platform, particularlymicromachined elements where SiO₂ and/or Si₃N₄ membranes are employed,since these materials are rapidly etched in the process environment towhich they are exposed to effect the sensing of the target gascomponent.

It would therefore be a significant advance in the art to provide amicromachined sensing device that is resistant to attack by the gasenvironment being monitored, e.g., where the gas environment to bemonitored contains fluoro species or other corrosive agents or etchants.

SUMMARY OF THE INVENTION

The present invention relates generally to apparatus and method forsensing fluoro species in an environment susceptible to the presence ofsuch species, such as an ambient environment, a gaseous effluent streamfrom a semiconductor manufacturing process, etc.

In one aspect, the invention relates to a gas sensor assembly comprisinga free-standing gas sensing element coupled on a substrate to means formonitoring a change in at least one property of the gas sensing elementupon contact thereof with a target gas species and responsivelygenerating a control signal, wherein the gas sensing element is formedof a material exhibiting said change in contact with the target gasspecies.

A further aspect of the invention relates to a solid state sensorcoupled in sensing relationship to a process chamber and arranged towithstand a corrosive condition within such process chamber.

Another aspect of the invention relates to a gas sensor assemblyarranged to monitor a effluent from a semiconductor manufacturing plantor a fluid derived from the effluent, wherein the effluent or fluidderived therefrom is susceptible of comprising a target gas species, andthe gas sensor assembly comprises a free-standing gas sensing elementcoupled on a substrate to means for monitoring a change in at least oneproperty of the gas sensing element upon contact thereof with the targetgas species in the effluent or a fluid derived from the effluent, andresponsively generating a control signal, wherein the gas sensingelement is formed of a material exhibiting such change in contact withthe target gas species.

A still further aspect of the invention relates to a method ofmonitoring a fluid locus for the presence of a target gas speciestherein, said method comprising:

exposing fluid at said fluid locus to a free-standing gas sensingelement formed of a material exhibiting a change in at least oneproperty of the gas sensing element upon contact thereof with the targetgas species;

monitoring said at least one property of the gas sensing element duringstep (a); and

responsively generating an output signal when the gas sensing elementexhibits said change in at least one property of the gas sensingelement, indicating the presence of the target gas species in the fluidlocus, or a change in concentration of the target gas species in thefluid locus.

In another aspect, the invention relates to a method of manufacturing agas sensor assembly, comprising the steps of:

providing a base assembly including a substrate member havingspaced-apart upstanding contacts thereon;

depositing a layer of support material on the base assembly between thecontacts;

depositing on the layer of support material a layer of a sensormaterial; and

removing support material under the layer of sensor material, to form afree-standing sensor material structure.

A further aspect of the invention relates to a method of manufacturing agas sensor assembly, comprising the steps of:

providing a substrate member;

forming a trench in the substrate member;

depositing a support material in the trench;

depositing a layer of a sensor material over the trench and adjacentsurface regions of the substrate member; and

removing support material from the trench under the layer of sensormaterial, to form a free-standing sensor material structure overlyingthe trench.

Another aspect of the invention relates to a gas sensor assemblycomprising a free-standing gas sensing element coupled on a substrate tomeans for monitoring a change in at least one property of the gassensing element upon contact thereof with a fluoro species andresponsively generating an output signal, wherein the gas sensingelement is formed of a material exhibiting such change in contact withthe fluoro species.

A further aspect of the invention relates to a solid state sensorcoupled in gas sensing relationship to a process chamber and arranged towithstand a corrosive condition within such process chamber, wherein thesolid state sensor comprises a free-standing gas sensing elementarranged for contacting the corrosive environment and responsive to thecontacting by change of at least one monitorable property of the gassensing element, and a signal generator arranged to output a signalindicative of the change in such at least one property of the gassensing element.

An additional aspect of the invention relates to a gas sensor assembly,comprising a substrate having deposited thereon a barrier layer forprotection of the substrate from attack during gas sensing, a layerdeposited on the barrier layer of a sensing material producing, inexposure to gas to be sensed in the gas sensing, a change in at leastone property or response characteristic of the sensing material layer,and a cavity formed in the substrate member on a back side thereof, suchcavity terminating at a back face of the sensing layer.

In a further aspect, the invention relates to a method of making a gassensing assembly, comprising:

providing a substrate member;

depositing a barrier layer on the substrate member;

depositing a sensing layer on the barrier layer; and

micromachining a backside cavity in the substrate member terminating atan interior face of the barrier layer.

Yet another aspect of the invention relates to a gas sensor assemblyincluding a free-standing metal sensor element arranged for selectiveresistance heating of the element and exhibiting a change in at leastone property of the element in contact with a fluoro species in agaseous environment, and a signal generator operatively coupled with thesensing element to output a signal indicative of presence of a fluorospecies in gas being monitored when the gas being monitored is contactedwith the sensing element and the gas being monitored contains suchfluoro species.

A still further aspect of the invention relates to a gas sensor assemblyincluding an array of posts, and one or more free-standing metal sensorwire(s) woven about such posts to provide a woven wire structure forcontacting with gas susceptible to presence of one or more targetspecies therein with which the wire is interactive to produce a responseindicative of the presence of the one or more target species.

In another aspect, the invention relates to a gas sensor assemblycomprising a micro-hotplate structure including a free-standing gassensing element responsive to presence of fluoro species by responseindicative of presence or increase in concentration of such fluorospecies. A further aspect of the invention relates to a gas sensordevice for detecting fluoro species in a gas environment, comprising afluoro species-resistant polyimide support structure and sensing wiresupported thereon for contacting the gas environment, wherein thesensing wire responsively exhibits a monitorable change in exposure tothe fluoro species.

A still further aspect of the invention relates to a gas sensingassembly comprising a free-standing gas sensing element responsive toexposure to fluoro species by a response indicative of said fluorospecies, wherein said free-standing gas sensing element comprises acomposite filament including a filament core having a fluorospecies-sensitive material coated thereon, wherein said core materialhas a higher resistivity than said fluoro species-responsive material.

Yet another aspect of the invention relates to a gas sensor assemblycomprising a free-standing gas sensing element coupled to connector pinsof a microelectronic device package, wherein the free-standing gassensing element is arranged for contact with a gaseous environmentsusceptible to the presence or change of concentration of one or moretarget gas species therein, and the free-standing gas sensing element isformed of a material that in exposure to the target gas species exhibitsa response transmissible through the connector pins of themicroelectronic device package.

Another aspect of the invention relates to a gas sensor assemblycomprising a free-standing member on a substrate, wherein thefree-standing member comprises a gas sensing element arranged forcontact with a gaseous environment susceptible to the presence or changeof concentration of one or more target gas species therein, and the gassensing element is formed of a material that in exposure to the targetgas species exhibits a response indicative of the presence or change ofconcentration of the one or more target gas species in the gaseousenvironment, with the free-standing member comprising a barrier layer ofa material resistant to the target gas species, supporting the gassensing element.

In a still further aspect, the invention relates to a gas sensorassembly comprising a free-standing gas sensing wire element woven ontoor into an insulative scaffolding member and comprising a multiplicityof windings thereon to form a woven wire structure, wherein the wireelement is formed of a material exhibiting a response in exposure totarget gas species, and such wire element is coupled to circuitry toproduce an output indicative of presence or change of concentration oftarget gas species in a gaseous environment when the wire element isexposed to the target gas species.

As used herein, the term “fluoro species” is intended to be broadlyconstrued to encompass all fluorine-containing materials, includingwithout limitation, gaseous fluorine compounds, fluorine per se inatomic and diatomic (F₂) forms, fluorine ions, and fluorine-containingionic species. The fluoro species may for example include species suchas NF₃, SiF₄, C₂F₆, HF, F₂, COF₂, ClF₃, IF₃, etc., and activatedfluorine-containing species (denoted collectively as F^(•)) thereof,including ionized fragments, plasma forms, etc.

Other aspects, features and embodiments of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DPAWINGS

FIGS. 1, 2 and 3 are schematic illustrations depicting the process flowin the manufacture of a sensor according to one embodiment of thepresent invention.

FIG. 4 depicts successive steps (Steps A through F) in the fabricationof a sensor assembly according to another embodiment of the invention.

FIGS. 5-8 depict successive steps in the fabrication of a sensorassembly with a chemically resistant barrier layer as a membrane supportmaterial.

FIG. 9 is a schematic representation of a gas sensor assembly, includingsensor wire woven in a vertically-oriented plane.

FIG. 10 is a schematic representation of a gas sensor assembly,including sensor wire woven in a horizontally-oriented plane.

FIG. 11 is a schematic representation of a sensor wire wrapped aroundposts in a racetrack pattern.

FIG. 12 shows a figure-eight pattern of sensor wire woven on supportposts.

FIG. 13 shows an “S”-shaped conformation of sensor wire wrapped onsupporting posts.

FIG. 14 shows a vertical weave gas sensor assembly utilizing a racetrackweaving technique.

FIG. 15 shows a woven gas sensor assembly having an “S”-shaped weave.

FIG. 16 shows a figure-eight conformation of weaving in a gas sensoraccording to another embodiment of the invention.

FIG. 17 is a top plan view of the gas sensor assembly of FIG. 14.

FIG. 18 is a top plan view of the gas sensor assembly of FIG. 15.

FIG. 19 is a top plan view of the gas sensor assembly of FIG. 16.

FIG. 20 illustrates a gas sensor assembly including a Vespel® polyimideblock element machine with orthogonal cuts forming shelves forcontrolling the vertical position of sensing wire.

FIG. 21 depicts a gas sensing assembly including a flange and Vespel®polyimide block element machined with complete parallel cuts to formposts about which sensor wires can be woven.

FIG. 22 illustrates a foraminous Vespel® polyimide element array andsensing wire assembly.

FIG. 23 shows a cylindrical Vespel® polyimide element with sensor wirewoven in a spiral array.

FIG. 24 illustrates a cylindrical Vespel® polyimide element providing asupport structure for vertically woven sensing wire.

FIG. 25 shows a cylindrical Vespel® polyimide element and an “S”-shapedweaving pattern of sensing wire.

FIG. 26 is a schematic diagram of an NF₃ plasma test manifold.

FIG. 27 is a graph of voltage drop and SiF₄ concentration from FourierTransform Infrared (FTIR) monitoring of a baseline Pt foil test.

FIG. 28 is a graph of voltage drop and SiF₄ concentration from FTIRmonitoring of the Pt foil when a silicon chip was added.

FIG. 29 is a graph of both the SiF₄ concentration and the change in Ptresistance, as a function of time, with the SiF₄ concentration measuredusing an FTIR on the exhaust line.

FIG. 30 is a side view of a machined Vespel® polyimide sensor assemblyaccording to another embodiment of the invention.

FIG. 31 is a top view of the sensor array of FIG. 30.

FIG. 32 is a side view of a machined Vespel® polyimide sensor assembly,according to yet another embodiment of the invention.

FIG. 33 is a top view of the sensor assembly of FIG. 34.

FIG. 34 is a side view of a machined Vespel® polyimide sensor assemblyaccording to a further embodiment of the invention.

FIG. 35 is a top view of the sensor assembly of FIG. 34.

FIG. 36 is a graph of end point monitor behavior, for a gas sensingassembly constructed in accordance with a still further embodiment ofthe invention, shown in the upper portion of the graph, and residual gasanalyzer concentration, shown in the lower portion of the graph, as afunction of time, in minutes.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

While the invention is described more fully hereinafter with specificreference to applications in semiconductor process control, it is to beappreciated that the utility of the invention is not thus limited, butrather extends to a wide variety of other uses and applications,including, without limitation, deployment in life safety systems, roomor ambient environment monitoring operations, and other industrial aswell as consumer market gas sensing applications.

The present invention in one aspect thereof providesmicroelectromechanical system (MEMS)-based gas sensing capability fordetermining the endpoints of semiconductor chamber clean processes.MEMS-based sensing has not been commercially viable in such application,prior to the present invention, due to two major challenges, viz., (1)the development of thin film materials that will have a measurableresponse to the heavily fluorinated gases (typically, NF₃, SiF₄, C₂F₆,HF, and activated species thereof) that typically are employed insemiconductor chamber cleaning, and (2) the integration and packaging ofsuch sensing films in a reliable form that will survive the harshenvironments of such heavily fluorinated gases.

These have been formidable challenges, since current MEMS designs (forother, more benign gas environments) require deposition of the sensingmetal layers on a silicon-based device structure, and subsequent bondingand packaging of the device into a chip carrier. This currentfabrication approach entails a multi-step process, involving acorresponding multicomponent product sensor assembly in which eachcomponent is subject to chemical attack by the heavily fluorinatedgases. While it may be possible to protect each of the respectivecomponents by developing a suitable encapsulation structure, suchexpedient adds further fabrication complexity, manufacturing time andcost to the product gas sensor device.

The present invention overcomes these obstacles in a manner enabling theuse of a MEMS-based sensor device that is easily and inexpensivelyfabricated, and readily implemented for monitoring fluorinated gases insemiconductor chamber clean processes in an efficient, durable andreliable manner in the harsh chemical environment of such processes.

The fluorinated gas sensor device of the invention, as described morefully hereinafter, has two primary features that distinguish it as abreakthrough in the art. One such feature is the use in the device offree-standing metal elements, functioning as a sensing material and asoptionally as a heat source (e.g., by resistive, conductive, or otherheating thereof) for the gas sensing operation, as for example where itis desired to vary the sensing temperature from ambient conditions, orto match the temperature of a semiconductor chamber whose effluentincludes the target gas species to be monitored. The second such featurerelates to the packaging of the free-standing metal films, wherein thefree-standing structure is able to be fabricated directly onto astandard chip carrier/device package so that the package becomes theplatform of the device.

The invention thus provides a solid-state sensor that can be coupled insensing relationship to a process chamber, e.g., a semiconductor processchamber, and can withstand a corrosive environment within the processchamber by appropriate selection of materials and sensing elements, ashereinafter more fully described.

The fluoro species sensor device of the invention may include a singlesensing element in any of the numerous suitable forms describedhereinafter.

Alternatively, the fluoro species sensor device may comprise a pluralityof such sensing elements, wherein the multiple elements provideredundancy or back-up sensing capability, or in which different ones ofthe multiple sensing elements are arranged for sensing of differentfluoro species in the stream or gas volume being monitored, or in whichdifferent ones of the sensing elements in the array are operated indifferent modes, or in interrelated modes, such as for production ofrespective signals that are algorithmically manipulated, e.g.,subtractively, to generate a net indicating signal, or alternatively,additively to produce a composite indicating signal, or in any othersuitable manner in which the multiplicity of sensor elements isefficaciously employed to monitor the flow of species in the stream orfluid volume of interest, for generation of correlative signal(s) formonitoring or control purposes.

As is well known, fluorine reacts with most metals, and gives rise tocompounds that have a high, and sometimes, mixed oxidation state(Inorganic Solid Fluorides, Chemistry and Physics. Academic Press, 1985,Ed P. Hagenmuller). Many of the transition metals and noble metals(including, for example, but not limited to, Ti, V, Cr, Mn, Nb, Mo, Ru,Pd, Ag, Ir, Ni, Al, Cu and Pt) readily form various non-volatilefluorinated compounds in contact with fluorine gas components. The gassensing device and method of the present invention use free-standingforms of these metals to detect the presence of fluorinated species inthe gas being monitored.

Detection of the fluoro species of interest may be achieved in anysuitable manner, e.g., by means of a change in resistance of thefree-standing metal material as it reacts with fluorine-containingspecies.

It will be recognized that the choice of a specific material offabrication for the free-standing fluoro species sensing element mayvary in the preferred practice of the invention according to thecharacter of the stream being monitored for the presence of fluorospecies, and particularly according to the corrosivity of the target gasspecies being monitored or otherwise present in the monitored gas, andthe corresponding corrosion-resistance of the sensing element materialin such exposure.

For example, palladium in some aggressively corrosive environments maybe less preferred in consequence of its etching by the monitored medium,than other sensing element materials of construction.

The choice of a specific sensing material of construction may be readilydetermined for a given end-use application of the invention, by simpleexperiment involving exposure of candidate gas sensing element materialsof construction to the fluoro species-containing environment, anddetermining the suitability, e.g., corrosion-resistance oretch-resistance, of the candidate materials in such exposure.

The free-standing sensing element in fluorine detectors of the inventionmay be provided in any of numerous suitable forms, including, but notlimited to, wires, filaments, foils, nanoporous free forms or coatings,electroplated metal, e.g., metal co-electroplated with liquid crystals,or thin films suspended above an air gap. These sensing elements mayhave tailored morphology, such as roughened surfaces, standardnanoporosity, or induced nanoporosity. In one embodiment of theinvention, electroplated nickel is utilized as the fluoro speciessensing material, and electroplated aluminum may also be employed toadvantage, as well as electroplated forms of any other of the previouslymentioned metals, as well as metals other than those specificallyillustratively mentioned. The reaction of the fluorine compound with thefree standing metal may be temperature-sensitive, and heating of themetal can be achieved by passing current through it. In this way, thegas sensing elements may be utilized in the gas sensing operation asself-heating structures, such character being enabled by thefree-standing nature of the structure.

The resistance and behavior of the free-standing metal can be engineeredby altering the geometry of the structure. For example, a free-standingwire can be thinned in any of a variety of ways, e.g., mechanically,chemically, electrochemically, optically or thermally, in order toincrease the absolute resistance, as well as to increase the surfacearea-to-volume ratio of the metal, to thereby increase the sensitivityor improve the signal-to-noise ratio. Likewise, the geometry of asuspended thin film can be engineered by choosing the width, length andthickness of the film over the suspended area appropriately. Further,the material's physical properties can be engineered. For example, thecomposition can be modified either by alloying or doping, and themicrostructure can be modified, e.g., by change in grain size, level ofcrystallinity, porosity (e.g., nanoporosity), surface area-to-volumeratio, etc.

It will therefore be apparent that the free-standing metal structure maybe variously configured and modified as desired with respect to itsform, conformation, physical properties, chemical properties andmorphological character, within the skill of the art and without undueexperimentation.

As discussed hereinabove, the free-standing metal structure of thesensor device of the invention can be readily fabricated directly onto astandard chip/carrier package, so that the package is effectivelyconstituted as the platform for the device. This packaging is animportant feature of the present invention in application tosemiconductor process gas monitoring devices, since the heavilyfluorinated environment characteristic of such semiconductormanufacturing application is an environment that is antithetical to theuse of conventional MEMS-based gas sensor devices. The gas sensingdevice of the invention, by use of a free-standing metal structure asthe sensing element that is integrated directly into a package,overcomes the susceptibility to chemical attack that has limited theability of the prior art to use MEMS-based sensor devices in suchapplications.

The free-standing structure can be integrated as part of the devicepackage in any suitable manner. For example, in the case of afree-standing wire sensing element, or a foil structure sensing element,the wire or foil structure can be spot welded directly to the packagingposts. The free-standing wire or foil can then be thinned-down in anysuitable manner, e.g., mechanically, chemically, electromechanically,electrochemically, thermally, optically, etc. A preferred thinningtechnique involves laser micro-machining of the free-standing metalsensing element.

As another illustrative approach for integrating the free-standing metalsensing element into the device package, an insulation layer may beapplied to the package for the device, followed by planarization of thisinsulator to expose the package pads, followed by thin film depositionto form the gas sensing element.

The thin film deposition may be carried out in any suitable manner, butpreferably it is effected by physical vapor deposition, and mostpreferably it is achieved by sputtering or e-beam evaporation. A shadowmask may be employed to delineate the structure of the deposited film.The insulation layer material may be organic or inorganic, but isadvantageously a material that can withstand the environment in which itused, i.e., it should be vacuum-compatible, etch-resistant, andnon-contaminating. This integrated structure can be further modified asdesired, e.g., by laser micro-machining. Laser micro-machining, forexample, can be used to further thin the geometry as well as to etchaway the insulating material, creating an air gap and thus yielding afree-standing thin film device structure.

The ability to integrate the free-standing structure into a standardmicroelectronic device package such as a chip carrier package, enablesthe gas sensor apparatus of the invention to be variously configured asa single-element device structure, or alternatively as a multi-elementarray, e.g., using varied metal structures, different geometries, orredundant structures operating at different temperatures, to enhance thegas detection capability of the overall sensing device. The number ofpins (contact structures) in the device package is a limiting factor indetermining the maximum size of the array, and the ready commercialavailability of a wide range of multi-pin device package structuresthereby correspondingly enables varying-sized arrays to be provided.

In instances where multiple metal sensing element structures areprovided, different ones of the multiple metal structures may beconstructed and arranged for sensing of different fluorinated species inthe fluid environment being monitored, and/or same fluorinated speciesat different temperatures, and different geometries and configurationsof sensing elements may be employed for redundancy and/or ensuringaccuracy, etc. Alternatively, or additionally, different ones of themultiple sensing elements may be operated in different operating modes,e.g., resistively, conductively, pulsed, a DC mode, an AC mode, etc.

In connection with the use of arrays of gas sensing elements, advanceddata processing techniques can be used to enhance the output of thesensor system. Examples of such techniques include, but are not limitedto, the use of compensating signals, the use of time-varying signals,heater currents, lock-in amplifying techniques, signal averaging, signaltime derivatives, and impedance spectroscopy techniques. In addition,advanced techniques that fall into the category of chemometrics may alsobe applied. These techniques include least squares fitting, inverseleast squares, principal component regression, and partial least squaredata analysis methods.

The gas sensing element(s) of the sensor assembly of the invention maytherefore be coupled in a suitable manner, within the skill of the art,to transducers, computational modules, or other signal processing units,to provide an output indicative of the present or change in amount ofone or more fluoro species in the fluid environment being monitored.

Referring now to the drawings, FIGS. 1, 2 and 3 are schematicillustrations depicting the process flow in the manufacture of a sensoraccording to one embodiment of the present invention, including thepackage fabrication of free-standing thin film elements to yield a gassensor assembly.

As shown in FIG. 1, a base assembly, including substrate member 1 havingcontacts 2 and 4 thereon, which may for example comprise a TO-5 or TO-8header, is first coated with plasma-resistant polymer 6, such aspolyimide, or a commercially available SU-8 photoresist, between thecontacts.

The excess material of the layer of polymer 6 then is polished off asshown in FIG. 2 by a suitable planarization step, to generate a flatsurface for subsequent e-beam evaporation of sensing metal.

A thin film 8 of Pt or other suitable metal then is formed on the layerof polymer 6 by a suitable technique such as e-beam evaporation of themetal onto the contacts (and polymer), e.g., using a shadow mask (notshown in FIG. 3). Additional processing may include laser trimming tofurther modify the thin film shape, and to remove polyimide material inselected regions from under the thin film structure, e.g., with a laserablation tool, to form a trench under the thin film gas sensor element.

The sensor assembly may thus comprise a thin film metal strip disposedover a surface of a sensor platform constituted by a Vespel® polyimideflange or electrical feedthrough with a polyimide protective layer. Ingeneral, the sensor assembly may employ any number of flanges, e.g., KFflanges, and such flanges may be formed of a suitable material such asVespel® polyimide (commercially available from E.I. du Pont de Nemoursand Company, Wilmington, Del.) or aluminum. In one embodiment, thesensor assembly fabrication includes press-fitting of TO heads intoVespel® polyimide flange members. Vespel® polyimide is a preferredpolyimide material of construction in various embodiments of theinvention as hereinafter more fully described, but it will be recognizedthat other polyimide or polymeric (e.g., polysulfone) materials ofconstruction may alternatively be used.

In a preferred aspect of the present invention that avoids the problemsof controlling the dimensions of the trench when laser ablation isemployed to form a trench beneath the sensor metal film, and relatedissues of minimizing damage to the sensor element metal layer andavoiding redeposition of substrate material on the sensor element, thefabrication process methodology shown in FIG. 4 may be carried out toform a gas sensor assembly according to another embodiment of theinvention.

FIG. 4 illustrates successive steps (Steps A through F) in thefabrication of the sensor assembly, in which each step is depicted withreference to a cross-sectional elevation view of the structure beingfabricated on the left-hand side of the figure, and the arrow labeledwith the step designation (see arrows A, B, C, D, E and F, in sequence,from the top to the bottom of the figure) connecting suchcross-sectional elevation view with a corresponding top plan view of thestructure, wherein the top plan view for each step is labeled with thereference numerals discussed in the ensuing description of the process.

In the FIG. 4 process, the substrate 10 is a Vespel® polyimide flange(Step A).

Laser drilled trenches 12 are formed in the Vespel® polyimide flange 10,producing the structure shown in Step B. While laser ablation is apreferred technique for such trench formation, other non-laser methodscould be employed, e.g., selective chemical etching of the substrate,RIE techniques, etc.

A layer of sacrificial material 14 then is applied over the surface ofthe laser-drilled substrate (Step C) so that the trenches are filledwith the sacrificial material. The sacrificial material preferably is amaterial that can be removed in subsequent processing by liquid- orgas-phase etching or other removal process. Examples include a polymericmaterial that is removable by ashing in the presence of O₂, or materialssuch as SiO₂ that are removable by a fluorine-containing plasma etch, orby appropriate chemical solution or solvent dissolution media.

Next (Step D), the surface of the structure is polished in aplanarization operation, to remove the sacrificial material 14 from thesurface of the flange surrounding the trenches, leaving trench deposits16 of the sacrificial material. The planarization step improves theplanarity of the substrate surface, and assists thereby in achievinggood control of the geometry of the subsequently formed metal element.The planarization step is optional, and may be omitted in instanceswhere good self-leveling behavior is achieved for the sacrificialmaterial and it is possible to apply the sacrificial material into thetrenches so as to be near-level with the adjacent surface of thesubstrate surrounding the trench cavity.

A sensing metallization material is then deposited (Step E) over thetrench deposit 16 and top surface portions of the substrate adjacent thetrench, to define the sensing element 18, as illustrated. The metalsensing element may be formed by deposition through a shadow mask, oralternatively as a blanket layer for subsequent patterning byphotoresist and etching.

Finally (Step F), the sacrificial material is removed from the trenches12, by etching or other suitable technique (solubilization, oxidativeashing, sublimation, etc.), so that the sensing element 18 overlies thetrench 12 as an exposed thin film element that can then be coupled byleads to suitable electronics, e.g., an electronics control moduleincluding power supply and signal processing componentry (not shown inFIG. 4).

Such electronics are suitably arranged so that one or more properties ofthe sensing element 18 can be monitored. When the monitored property,e.g., electrical resistance or other suitable property of the metalsensor element, changes as a result of interaction with the fluorospecies for which the metal sensor element is sensitive, the electronicsprovide a correlative output, e.g., a control signal, visual displayoutput, etc., indicative of the presence or amount of the target gasspecies in the environment being monitored.

In an illustrative embodiment, the output may be a control signal thatis employed to modulate a process from which the monitored gas isobtained. In a semiconductor manufacturing operation, such output mayactuate a central processing unit (CPU), microprocessor, or other signalprocessing or signal-responsive means, to switch the process valves, andterminate a processing operation, or initiate a new process step orcondition.

For example, on being contacted by fluorine compound(s) such as SiF₄,and/or other fluoro species, the voltage across the metal sensingelement (as a component of an electrical circuit) may drop, indicativeof an increase in resistance of the metal sensing element incident toits contact with a target fluoro species. Such voltage drop can beemployed to generate a signal for process control purposes. The voltagedrop can be employed to generate a signal that actuates an automaticcontrol valve, to effect flow initiation, flow termination, or flowswitching of a process stream in the semiconductor process system. Thecontrol signal alternatively may be employed to actuate a cycle timer,to initiate a new step in the process operation, or to signal that amaintenance event, such as change-out of a scrubber resin in anabatement process chamber, is necessary or desirable.

It will be appreciated that the change in properties of the metalsensing element can be exploited in any of a variety of ways, to effectthe control of a process in relation to the sensing of the target gas(e.g., fluoro) species, within the skill of the art and without undueexperimentation.

By way of further examples, the sensor assembly of the invention may beutilized in connection with a gas cabinet containing a supply of afluoro species gas (such as a perfluoro species, e.g., a perfluorinatedorganometallic precursor for chemical vapor deposition operations), andthe gas sensor assembly may be employed to determine the existence of aleak from the supply vessel or otherwise in the flow circuitry in thegas cabinet. The sensing of the fluoro species then may be utilized toactuate a source of bulk purge gas, to sweep out the interior volume ofthe gas cabinet and prevent the concentration of the fluoro species fromreaching toxic or otherwise hazardous levels.

The sensor assembly may also be utilized in a monitoring unit for anambient environment that is susceptible to the ingress or generation offluoro species therein, or alternatively the sensor assembly could be aconstituent part of a wearable gas monitoring unit that is arranged toactuate an alarm and/or a self-contained source of emergency breathinggas, for hazardous materials cleanup crews, firefighters in chemicalcomplexes, workers in HF glass-etching operations, etc.

The metal that is employed in the metal sensor element of the presentinvention may comprise any suitable metal species that in exposure tothe target gas species, e.g., one or more target fluoro species,produces a change that is monitorable and useful as an indicator of suchtarget species (e.g., the presence of such target species, or changes inconcentration of such target species).

Examples of metal sensor elements that may be employed for fluorospecies sensing in the broad practice of the present invention, include,but are not limited to, one or more of Ti, V, Cr, Mn, Nb, Mo, Ru, Pd,Ag, Ir, Ni, Al, Cu and Pt. The metal may be in the form of an alloy orit may comprise a combination of metals, and composite sensing elementsincluding a variety of metal species, or of metal and non-metal speciesin combination with one another, are contemplated within the broad scopeof the present invention.

The metal sensor element in the sensor assembly of the invention ispreferably of a high surface to volume (S/V) character, to facilitaterapid response, and to amplify the response relative to thesubstantially lower change in the gas-indicating bulk property thatwould otherwise occur in a low S/V conformation of the same sensormaterial.

Thus, preferred high responsivity forms of the sensor material includefoils, films, filaments, needles, powders, etc., as well as metal-dopedconductive threads, vapor-deposited metals on carbon nanotubes, and thelike. The critical dimension of the metal sensing element—the thicknessdimension for foils or films, or the diameter for forms such asfilaments, needles, powders, etc.—desirably is less than 500 microns(μm), preferably less than 150 μm, more preferably less than 25 μm,still more preferably is less than 10 μm, and most preferably is in arange of from about 0.1 μm to about 5 μm, as a balance of response speedand ease of fabrication considerations.

Foils and films, in addition to having a low thickness, e.g., in a rangeof from about 0.1 μm to about 50 μm, desirably have small dimensionalcharacteristics in the plane perpendicular to the thickness direction ofthe foil or film, again for reasons of responsivity. The lateraldimensions in such plane (x-y plane, where the z axis is the thicknessdirection) include a length (x-direction) and width (y-direction) thatare advantageously less than about 10 cm, preferably being less thanabout 1 mm and more preferably less than about 100 μm, e.g., in a rangeof from about 20 μm to about 5 mm, as a balance of fabricationalcomplexity and responsivity. The length of wires when used as a metalsensing element may be of any suitable length, particularly when used inwoven structures as herein described. As a specific example, wireshaving a length of 7-15 cm and a diameter in a range of 75 to 150 μm areusefully employed in one embodiment of the invention. In general,suitable dimensions of sensor wires can be readily determined to providecorrespondingly suitable signal-to-noise ratios for the intendedapplications.

In the context of the foregoing description, it is to be appreciatedthat the metal sensing element could be fabricated as a nano-scaleelement, albeit as a more costly gas sensor product than the typicallymillimeter/micrometer-scale elements discussed above.

The sensing element of the invention is a free-standing element, i.e.,it has a sensing portion that is exposed to the fluid environment thatis being monitored by the sensor for the presence of the target species,e.g., fluoro species, of interest, to maximize the sensitivity, responsetime and operating life of the sensing element.

In one embodiment, the free-standing gas sensing element may have afibrous or filamentary conformation, in which the end portions of theelongate gas sensing element are bonded or otherwise coupled to contactsor other circuit components, and the intermediate portion of the elementis unsupported and constitutes the free-standing section of the element,between its fixed ends. Correspondingly, the free-standing element maybe fabricated in a foil or film conformation, in which portions of thefoil or film are bonded or otherwise coupled to contacts or othercircuit components, with the region of the foil or film intermediate thecontacts or other circuit components being unsupported, and constitutingthe free-standing portion of the gas sensing element.

The packaging of the sensor device in a preferred aspect of theinvention is facilitated by the formation of the sensor device directlyon a standard chip carrier/device package, thereby simplifying theinterconnections of the device to the associated microelectroniccircuitry that enable the gas monitoring and control functions of thesensor device assembly. For example, electrical contact to the metalsensing element may be effected from the backside of the substrate bythrough-vias or pins disposed in appropriate locations to effect thenecessary electrical interconnection.

As is apparent from the foregoing discussion, the gas sensor assembly ofthe invention is readily fabricated in a simple and reproducible manner,and enables sensing of fluoro species to be achieved in a cost-effectivefashion, using conventional signal processing and control componentry towhich the gas sensor assembly of the invention can be convenientlycoupled.

The gas sensor assembly of the invention is readily applicable tomonitoring of fluoro species in various industrial process operationsgenerating such species, including semiconductor manufacturingoperations such as chamber cleans, in which fluoro species are utilizedfor removing silicon oxides, silicon nitrides, and low dielectricconstant (k<3.9) silicon-containing films such as carbon-doped siliconoxides, etc.

FIGS. 5-8 depict successive steps in the fabrication of a gas sensorassembly including a chemically resistant barrier layer as a membranesupport material, in accordance with another embodiment of the presentinvention.

FIG. 5 shows a substrate member 50, formed of silicon or other suitablematerial. As illustrated in FIG. 6, the substrate member 50 is shownsubsequent to deposition of a barrier layer 52 thereon. The barrierlayer 52 protects the substrate 50 from attack by the gas to bemonitored by the sensor, e.g., a gas containing one or more fluorospecies, or other target component(s). The barrier layer may be formedof a suitable inorganic dielectric material, such as silicon carbide,diamond-like carbon, etc. Alternatively, the barrier layer may be formedof an organic material, e.g., a polyimide.

Subsequent to formation of the barrier layer by suitable depositiontechnique or other fabrication method, a gas sensing layer 54 isdeposited, as shown in FIG. 7. The gas sensing layer may comprise asuitable metal, such as nickel, platinum, copper or aluminum, or othersuitable material exhibiting a change of a material property, orotherwise exhibiting a suitable response, in exposure to the fluorospecies or other target gas component to be sensed. The sensing layer 54may be deposited in any suitable form and manner, such as in the form ofa blanket layer for subsequent patterning by etching, or through ashadow mask.

A variety of designs is possible, and an array of devices of differentdimensions may be advantageously employed to maximize the efficiency ofthe gas sensor assembly, in respect of generation and outputting of aplurality of signals for the monitoring of the one or more target gasspecies in the fluid environment being monitored by the assembly.

FIG. 8 depicts the sensor assembly after micromachining the backside ofthe substrate member 50 to form the cavity 56 on the back side of theassembly. Such cavity may be formed by standard etching techniques, orother material removal process.

The sensor assembly shown in FIG. 8 has the advantage that itsfabrication avoids the provision of easily etched materials on the frontsensing side of the assembly. The thickness and characteristics of thebarrier layer 52 may be optimized by standard practices. Electricalcontact to the metal sensor layer 54 may be effected by wire bonding atthe top of the assembly, or through the barrier layer by buried contactsand through vias.

Although back-side etching is well-known for MEMS technology, the sensorassembly shown in FIG. 8 utilizes a chemically resistant barrier layer52 as a membrane support material for the sensing layer 54, and thusfundamentally varies from prior known sensor structures employed in theart.

The sensing assembly shown in FIG. 8 after formation of electricalcontacts may be inserted into a package from behind and sealed accordingto techniques known in the art. Alternatively, the sensor assembly aftercontact formation may be mounted on the front side of a robust flangematerial, e.g. a KF flange formed of Vespel® polyimide or aluminum.

The present invention thus provides microelectromechanical (MEMS) gassensor assemblies featuring free-standing metal sensor elements (i.e.,metal sensor elements that are structurally unsupported over a portion,preferably at least a major portion, of their length or physical extent)that are integrateably directly into a device package. The resulting gassensor assembly may be employed in a semiconductor manufacturingfacility for determining the end point of semiconductor chamber cleanprocesses (at the point of breakthrough of fluoro species or othertarget gas components in the effluent being monitored by the sensorassembly).

The fluoro species sensing elements used in gas detectors of theinvention suitably comprise metals that readily form non-volatilefluorinated compounds when exposed to fluoro species in the gasenvironment contacted with the sensing element, resulting in ameasurable change in electrical characteristics or other properties orresponse of the sensing element.

The free-standing architecture of the metal sensing element allows it tobe used both as a sensing material and a heat source (e.g., susceptibleto electrical resistance heating or other mode of heating), as well asmaximizing the sensing area, as a result of the high surface-to-volumecharacter of the sensing element, in the preferred forms previouslydescribed (foils, filaments, microparticulates, etc.). The integrateddesign of the sensing material and associated packaging obviates theproblem of chemical attack by aggressive fluorinated gas species in thesensing environment, thereby achieving a fundamental advance in the artover standard silicon MEMS structures.

The gas sensor assemblies of the invention in a preferred mode ofoperation exhibit a measurable change in resistance of the free-standingmetal sensing element as it reacts with fluorine-containing material inthe sensed environment. The dimensions of the free-standing metalsensing element are selected so that the resistance (or otherquantitatively measured response characteristic) is adequate fordetection of the target gas species in the monitored environment, withacceptable sensitivity and signal-to-noise characteristics.

Such criteria have posed a substantial challenge to the prior art, buthave been achieved in the gas sensor assembly of the present invention,by the provision of a free-standing metal sensor element having athree-dimensional architecture that produces a resistance or otherresponse characteristic of the desired magnitude, with concomitantincreased sensitivity and/or improved signal-to-noise ratio in relationto prior art gas sensor devices.

In a specific embodiment, the fluoro species (e.g., NF₃, SiF₄, F₆, HF,etc., and activated species thereof) gas sensing assembly of theinvention utilizes free-standing metal structures, e.g., wires, as thesensing element and heat source for the sensing operation, wherein themetal structures are integrated with microelectronic device packaging,by spot-welding directly to the packaging posts of the microelectronicpackage, e.g., a standard chip carrier package.

In such embodiment, the posts advantageously are arranged in an array,preferably such that the heads of the posts are aligned in a samehorizontal two-dimensional plane, with equal spacing between adjacentposts.

To ensure sufficient absolute resistance production by the gas sensorassembly, the length of the sensing element (wire) may be varied whilekeeping the wire diameter constant within manufacturing tolerances. Whenthe wire length is substantially larger than the distance between posts,the wire has a susceptibility to coil uncontrollably between anchoringpoints on adjacent posts. Such susceptibility is overcome using athree-dimensional packaging architecture in accordance with a furtheraspect of the invention, such that wire lengths may be made considerablylonger, to increase absolute device resistance and signal-to-noiseratios, while maintaining strict linear control of the wire position, ashereinafter described in greater detail. Various architectures may beutilized, in which metal packaging posts or machined Vespel® polyimideis employed to control wire position in three dimensions.

In an illustrative packaging post architecture according to oneembodiment of the invention, packaging posts formed of a suitable(electrically and thermally) insulating material are used as athree-dimensional framework around which sensor wires are woven. Wiresin such architecture are electrically contacted at their terminii toseparate posts, as for example, by spot-welding or by other suitablemethod of electrical contact such as press-fitting. Intermediate theanchored terminii of the wires, the wires are woven around the post. Theextent of weaving and the number of posts incorporated in thearchitecture can be selectively varied to enable the desired length ofthe sensing wire to be achieved.

Regardless of the method of weaving of the wire, two principal criteriamust be observed, viz., (i) that the wires not contact along their ownlength or with other wires, and (ii) that the wires not contact themetal of the posts except at the points of intended electrical contact.The second criterion necessitates that the posts be sheathed with aninsulating material except at the points of intended electrical contact.Posts in the architecture that do not function as intended electricalcontact points for any of the wires of the gas sensor assembly need notbe metal, but can be formed of Vespel® polyimide, or other suitablefluoro species-resistant insulating material.

A wide variety of techniques may be employed, consistent with theforegoing criteria, to achieve a desired wire length, and thus absoluteresistance, in the packaging post architecture. Two general categoriesof such techniques may advantageously be employed—those in which thesensor wire is woven around the posts to form a woven structure in avertically-oriented plane, as for example is shown in FIG. 9, in whichthe gas sensor assembly 60 comprises an array of posts 62 and the“vertical” weave 64 of sensor wire forms a woven structure in avertically-oriented plane, and those techniques in which the sensor wireis woven around the post to form a woven structure in ahorizontally-oriented plane, as for example is illustrated in FIG. 10,wherein the gas sensor assembly 70 comprises an array of posts 72featuring a “horizontal” weave 74 of sensor wire.

The specific method of weaving of the sensor wire within a horizontal orvertical plane may be additionally varied in the practice of theinvention.

For example, the sensor wire may be wrapped around the supporting postsin a racetrack pattern, as shown in FIG. 11, illustrating posts 80 and82 and the racetrack winding 84 or sensor wire, but care must be takento avoid forming a wire loop acting as a radio frequency inductor, sinceit thereby may be unsuitable for sensor usage due to spuriousinterference signals.

Other weaving patterns include the figure-eight pattern shown in FIG.12, wherein the posts 90 and 92 support a figure-eight conformation ofsensor wire 96, and the “S”-shaped weaving pattern shown in FIG. 13,wherein the posts 100 and 102 support the “S”-shaped conformation ofsensor wire 104.

In addition to such weaving conformations, any number of weavingarrangements may be employed within the skill of the art. A matrix maybe constructed for the purpose of representing possible permutations ofweaving that can be applied to the sensing devices. For example, avertical weave post assembly may be fabricated utilizing the “racetrack”weaving technique shown in FIG. 14, the “S”-shaped weaving techniqueshown in FIG. 15, or the figure-eight conformation shown in FIG. 16.

FIG. 14 depicts a gas sensor array 110 including a flange 112 supportinga Vespel® polyimide block 114, and an array of posts 116 fabricated ofmetal, as shown at the exposed post portions 118 below the flange, andthe insulating material-sheathed portions 120 of the posts above theVespel® polyimide block 114, as illustrated. The sensor wire 122 isillustrated as having a racetrack conformation, of a type as shown forexample in FIG. 11.

FIG. 15 depicts a gas sensor assembly 130 including a flange 132supporting a Vespel® polyimide block 134 and an array 136 of posts 138,of which exposed metal portions 140 are shown below the flange andinsulating material-sheathed portions 142 are shown above the Vespel®polyimide block, wherein the gas sensor wire 137 is woven in an“S”-shaped conformation, of a type as shown in the plan view of FIG. 13.

FIG. 16 depicts a gas sensor assembly 150 including a flange 152 andVespel® polyimide block 154 with an array of posts 156. The exposedmetal portions 158 of the posts are shown below the flange 152 and theinsulation-sheathed portions 160 of the posts are shown above theVespel® polyimide block 154. The wire weaving pattern of the sensor wire162 in FIG. 16 is of a figure-eight conformation, of a type asillustrated in FIG. 12.

FIGS. 17, 18 and 19 show top plan views of the gas sensor assemblies ofFIG. 14, FIG. 15 and FIG. 16, respectively.

The multi-post arrays of the gas sensor assemblies of FIGS. 17, 18 and19 are readily adapted to test various sensing wire materials ofconstruction, and in each of FIGS. 17-19, the far-right vertical row ofposts is shown as having the specific conformation of wire previouslydescribed. It will be appreciated that successive vertical rows(progressively left of the row shown with the sensor wire on the posts)may comprise wires formed of different sensing metals, whereby a matrixof weaving techniques and different sensing metals is provided. Such amatrix permits testing of each of the permutations to determine a mosteffective packaging post/sensing wire weaving design in a givenapplication of the present invention.

The gas sensor assembly of the invention in another aspect may utilize amachined Vespel® polyimide architecture. Vespel® polyimide is apolyimide having high dielectric strength, high heat resistance, highcompressive strength and excellent dimensional stability, and suchmaterial has been determined by the present inventors to possess a veryhigh resistance to attack by fluoro species. Vespel® polyimide maytherefore be used as a material of construction for a three-dimensionalframework for weaving of sensing wires.

Other fluorine-resistant solid materials may also usefully be employedin the broad practice of the present invention, but Vespel® polyimidehas been found to be highly efficacious and is preferred in this aspectof the invention.

Utilizing a three-dimensional framework for weaving of sensing wireswith a Vespel® polyimide structure, electrical contact of the sensingwires is made as in the previously described embodiments to the metalpackaging posts, however the metal posts may be relieved of their dualfunctionality requirements as electrical contacts and wire scaffoldingelements by concurrent use of a scaffolding support structure for thegas sensing wires or other free-standing gas sensing elements.

In such arrangements where electrical contact posts are used incombination with scaffolding structures to support free-standing wiresor other gas sensing elements, the metal posts do not require anyinsulation. Thus, intermediate anchored terminii, gas sensor wires maybe woven around insulative material structures such as Vespel® polyimidescaffolding members, and such scaffolding members may in turn be mountedon flanges or other substrate or support elements.

Vespel® polyimide is commercially available in block and cylindricalforms as well as in powder form, which may be pressure molded to anydesired shape. Subsequent machining of the block, cylinder or moldedVespel® polyimide material, provides structure(s) suitable forscaffolding of the sensing wire in a three-dimensional architecture.

Controlled weaving of the sensing wire into or around such scaffoldingusing the previously discussed techniques for packaging post-baseddesigns, allows the sensing wire to achieve necessary lengthcharacteristics, and corresponding absolute resistance required foreffective fluoro species sensing. The Vespel® polyimide material, orother fluorine-resistant solid material, may be machined in a fashionthat allows it to manipulate the position of the wires, as shown in thefollowing illustrative embodiments.

Block, cylinder or molded Vespel® polyimide material may be machined inany of various ways to produce structures on or into which the sensingwire may be woven. For example, cuts may be made into the Vespel®polyimide material to produce channels through which wires may be woven.Such cuts may penetrate completely through the material, or terminate ata prescribed depth, with the uncut material below the cut creating a“shelf” to control the vertical position of the wire. Additionally,vertical columns created on either side of a cut may act as “posts”around which the wires may be wound.

FIGS. 20 and 21 show respective Vespel® polyimide structures utilizingorthogonal vertical cuts with “shelves” as illustrated in FIG. 20, andparallel vertical cuts through the Vespel® polyimide block to createchannels and posts, as shown in FIG. 21. Wires may be woven in theseembodiments utilizing any of the various patterns previously described.The cut Vespel® polyimide device designs are easily adapted to includearrays of different or multiple sensing wires. As with packaging postsensor designs, a matrix may be fabricated with all possible cut Vespel®polyimide structures and all of the weaving patterns, such that sensorsof all desired design permutations are presented for assessment, andempirically-based selection of the best design for a given gas sensingapplication.

FIG. 20 illustrates a gas sensor assembly comprising flange 200 on whichis disposed Vespel® polyimide block elements 202, which together withmetal posts 204 provide support structure for weaving of sensor wires206 as illustrated. The Vespel® polyimide block elements are machinedwith orthogonal cuts with some incomplete cuts for the purpose ofconstructing shelves to control the vertical position of the sensingwire.

FIG. 21 depicts another gas sensing assembly including a flange 210 onwhich is disposed an array of Vespel® polyimide block elements 212 andposts 214 for support of sensing wires 216. The structure shown in FIG.21 features Vespel® polyimide block elements that are machined withcomplete vertically-oriented parallel cuts, creating posts between thesuccessive cuts, around which the wires can be woven. Both of the gassensor assembly structures illustratively shown in FIGS. 20 and 21utilize an “S”-shaped weaving pattern.

Gas sensor assemblies of the invention may also be fabricated in whichVespel® polyimide elements have holes drilled therethrough to create a“pegboard” structure through which wires may be threaded to produce anarchitecture for support of lengthy sensing wires.

Such approach is illustrated by the exemplary gas sensing assembliesshown in FIGS. 22-25.

In additional to utilizing Vespel® polyimide elements in the form ofplanar sheets of material with holes drilled therein, non-planar“pegboard”-type structures of other widely varied geometries may also beemployed as scaffolding elements for sensing wires. For example, bydrilling out the center of a Vespel® polyimide cylinder, a tube may beproduced. Further drilling of smaller holes along the sides of the tubecreates a cylindrical “pegboard”-type framework through which sensingwires can be threaded and supported such that the position of the wirealong its entire length can be controlled.

Many variations are possible for such “pegboard”-type scaffolding, whichmay be varied in respect of the numbers and dimensions/shapes of Vespel®polyimide support elements, the numbers and drilling patterns of holes,and the arrangements of threading and weaving the wire through suchholes. Arrays of sensing wires may easily be constructed using such“pegboard” architectures, and weaving styles and patterns may beselectively varied. Again, a matrix may be constructed of possiblepermutations of device designs, for purposes of assessing a bestpossible architecture for a given target gas species sensingapplication.

FIGS. 22-25 each include respective side, cross-section (except in thecase of FIG. 22), and top views of respective examples of sensor devicesutilizing machined Vespel® polyimide structures as “pegboard”-typescaffolding for fabricating sensing wire arrays.

FIG. 22 shows Vespel® polyimide elements 250 and 252 disposed on aflange 254, along with metal contact posts 256. The Vespel® polyimideelements 250 and 252 each contain a multiplicity of holes through whichsensing wire 260 is threaded (there is only a side view and top view ofthe gas sensor assembly in FIG. 22, whereas respective side view,cross-sectional view and top view representations are shown in each ofFIGS. 23-25). The arrangement shown in FIG. 22 provides ahorizontally-woven array utilizing an “S”-shaped weaving pattern.

FIG. 23 shows a cylindrical Vespel® polyimide element 270 in cooperationwith metal posts 272 on a flange 274, in which sensing wire 276 is wovenon the Vespel® polyimide element in a spiral array.

FIG. 24 shows another cylindrical Vespel® polyimide element 300 andmetal posts 302 on a flange 304 providing a support structure forvertically woven sensing wire 306, as illustrated.

FIG. 25 shows a cylindrical Vespel® polyimide element 310 in combinationwith metal posts 312 on a flange 314 for provision of an “S”-weavingpattern of sensing wire in a horizontally woven arrangement.

In another aspect, the invention contemplates arrays of micromachinedchemical sensor devices, wherein the micromachined chemical sensordevices have coatings comprising organic and/or inorganic sensormaterial that is reactive with the target species to be monitored. Thesensor coatings in exposure to a gaseous environment containing thetarget gas species yield physical, electrical, and/or other changes thatare indicative of the presence of the target species in the monitoredenvironment. Such micromachined sensor arrays are usefully employed forsensing of fluoro species in the monitored environment.

In a particular aspect described more fully hereinafter, the gas sensingdevices of the invention may include micro-hotplate sensing structures.

By using an array of sensors, multiple species of target components canbe detected. For example, if an array of four sensing elements isemployed, and each element has been designed to detect specific targetspecies, four such species can be detected simultaneously. If, as isfrequently the case, coatings that are specific to interaction with onlyone target species are difficult to produce, responses from multipledevices can be combined or otherwise algorithmically manipulated inreference to one another, to positively identify presence andconcentration of target species.

For example, the responses from six non-specific sensors can be combinedto produce an unambiguous identification of three target gas species.Additionally, sensors within a multi-element array can be individuallyoperated at different conditions, e.g., different temperatures, furtherexpanding the number of variables available to produce a unique sensingcapability.

Further, by using an array of sensing elements, a number of redundantsensors can be incorporated in one package. The array can be broughtinto use sequentially as regards component sensor elements, therebyexpanding the lifetime of the overall sensor assembly package, orpermitting the use of sensor coatings that are consumed in the sensingprocess.

As an example, an array may utilize a polymer coating to suppressreactions with sensors that are not being used. At the time of use, thetemperature of a particular array element may be selectively altered tomelt or bum away the polymer coating and expose a reactive coating.Additional elements can be brought on-line in a similar fashion, asneeded.

A number of advantages are inherent in the use of micro-machined sensorelements for fabricating a sensor array. These advantages include,without limitation, miniaturization of the sensor elements, ease ofarray fabrication, suitability for high-volume, low-cost manufacturing,low power consumption, and the ability to accommodate on-boardintegrated circuitry, which further lowers the size and cost of thefinal sensing array structure.

Although less preferred in the broad practice of the present invention,relative to other forms of the gas sensor assembly illustrativelydescribed herein, the gas sensor assembly of the invention may befabricated as a micro-hotplate sensor assembly, in which the gas sensingelement is deployed with a protective coating over the active fluorospecies sensing material.

Examples of illustrative film materials that may be employed fordetection of fluoro species include Cr, Cu, W, Ni, Al, and Si, as usefulmetal species, as well as polymeric materials. Such sensing elements maybe used in a chemical vapor deposition (CVD) process wherein nitrogentrifluoride (NF₃) is dissociated by a plasma to form reactive fluorinespecies.

F₂ or F reaching the MEMS sensor device reacts with the aluminum,nickel, tungsten, chrome, silicon or other active sensor material,resulting in a change in resistance of the material. This change inresistance will be a function of time, the operating temperature of thesensor material and the concentration of the fluorine-containingcompound or ionic species.

In a reversible device, the fluorinated reaction product, such as NF₂,would remain on the sensing material, but the resistance of the materialwould increase in proportion to the thickness of the fluorinated filmlayer. When the target fluorinated compound is removed from the gasstream being monitored, the reaction at the surface of the active layerwill be driven towards the metal. The initial resistance will thereby berecovered when the fluorine has been driven off.

In an irreversible process, the fluorinated reaction product, such astungsten hexafluoride, WF₆, will volatilize and be removed in the gasstream. The resistance of the active layer will increase as the tungstenis removed. The resistance will not decrease upon removal of thefluorinated compounds. The rate of removal/resistance increase will beindicative of the amount of fluorinated species present. In thiscircumstance, it may be advantageous to provide several devices in anarray, and to employ same sequentially, since they are consumed in thesensing process. A polymer or other organic film can effectively coatthe unused sensing elements, and can be melted or burned off, asrequired in the operation of the device.

In the case of organic coatings as sensing materials, a wide variety ofconductive polymers are commercially available. Examples of suchpolymers include, without limitation, polyphenylene vinylene, a crosslinkable monomer cast from methanol, and octylthiophene, awell-characterized and readily commercially available material.

In general, gas is desorbed from polymers more slowly than it isadsorbed. Accordingly, adsorption/desorption schemes are enhanced by theability of the micro-hotplate to ramp up the substrate temperaturequickly, to aid in effecting rapid desorption of adsorbed gaseousspecies.

Adsorption of acidic species enhances the conductivity of conductivepolymers by several orders of magnitude. Adsorption of species such ashydrogen chloride, which is present during an oxide chamber clean in asemiconductor manufacturing operation, will affect the conductivity of apolymer such as octylthiophene. Since HF is the reaction product ofatomic fluorine with hydrogen found in the oxide, hydrogen fluoride mayalso be used as an indicator of the progress of the chamber cleanprocess. The polymer conductivity may correspondingly remain constant orreverse as the level of hydrogen fluoride decreases, indicating thetermination of the chamber clean process.

Alternatively, the sensing film, e.g., formed of octylthiophene, mayreact with fluorine present during the oxide chamber clean, causing adrastic reduction in the conductive properties of the polymer as thechemistry is changed by reaction of the polymer with fluorine.

It will be recognized that micro-hotplate embodiments of the gas sensingassembly of the present invention may be widely varied in respect of thecomponent sensing films and reactive/sorptive chemistries employed, asdeterminable within the skill of the art for a given end use applicationof target gas species detection. Micro-hotplate detectors of a typeadaptable to the practice of the present invention may be fabricated asmore fully described in U.S. Pat. No. 6,265,222 issued Jul. 24, 2001 inthe names of Frank DiMeo, Jr. and Gautam Bahndari, the disclosure ofwhich hereby is incorporated herein by reference in its entirety.

In one presently preferred sensing element embodiment, wherein thesensing element is in the form of a filament, the sensor filamentcomprises a Monel core, e.g., having a diameter on the order of about100 microns, which is nickel-plated. In lieu of Monel, other corematerials, such as iron-nickel alloys, stainless steels, indium,vanadium and cobalt alloys, etc., may be employed for such compositewire sensor element.

Nickel is highly resistive in character, and Monel has an even higherresistivity. Their combination therefore enables maximization of thesensing signal with a large resistance, so that higher responsivity andeffective signal generation are achieved.

In such core and cladding composite filament structures, the corematerial may comprise a material that would not itself be suitable as asensor element, e.g., due to alloy contaminants or other considerations,but which has a high resistivity that enables another material, such asnickel, when plated thereon, to compensate for the deficiencies of thecore material, and provide an overall high signal, high resistivitycomposite sensor element.

The details, features, and embodiments of the invention are more fullyillustrated with respect to the following non-limiting examples.

EXAMPLE 1

A platinum foil having a thickness of 4 micrometers (μm) was connectedvia spot welding to the four contacts of an 8-lead TO-5 header. Lasermicro-machining was used to form a narrow, higher resistance area, withan approximate size of 80 μm×50 μm. The room temperature (˜25° C.)resistance of the foil between the middle two contacts was 0.14 Ω. Thisfoil, as reconfigured to a filament form, was the Pt test sample.

The NF₃ plasma test manifold used in this test is illustratedschematically in FIG. 26, as including silicon wafers 30 in the manifoldflow circuitry (which however were provided in the flow circuit only inthe semiconductor chamber clean process simulation test and not in thebaseline run), upstream of the Pt test sample 32.

The manifold flow circuitry included a flow conduit joining the plasmagenerator 34 with a downstream main vacuum pump 36, which in turn wasjoined to a dry scrubber unit 38. The distance from the juncture of theplasma generator 34 with the manifold conduit to the Pt test sample 32was 28 inches. The manifold conduit between the main vacuum pump 36 andthe dry scrubber 38 was equipped with taps to which an FTIR gas cell 20was coupled, via associated gas flow tubing, as shown. The direction offlow in the test manifold is shown at respective positions in the flowcircuit by arrows labeled “flow direction.”

A free-standing Pt foil was installed in the NF₃ plasma test manifoldillustrated in FIG. 26, and exposed to various fluorinated species. Theplasma test was conducted at a pressure of about 5 torr. A constantcurrent (I) of 50 mA was run through the foil and the voltage change(ΔV) measured as a function of time. The SiF₄ concentration was measureddownstream of the main vacuum pump 36, using the FTIR gas cell 40, alsoas a function of time.

When the foil was exposed to the baseline condition (without Si or SiO₂present), the voltage drop in the foil increased in a manner similar tothe SiF₄ concentration, as shown in FIG. 27.

NF₃ plasma was generated by the plasma generator 34 and flowed into themanifold in the baseline test as well as in the subsequent chamber cleanprocess simulation test. NF₃ plasma is known to attack silicon and SiO₂that may be present on the sidewalls of a semiconductor processingchamber. The by-product of the NF₃ plasma includes volatile SiF₄. Whenthe chamber is clean, the concentration of SiF₄ decreases. To simulatethis occurrence in the test manifold, silicon wafers 30 were provided inthe manifold to generate SiF₄.

The voltage drop across the exposed Pt foil element as a function oftime, and the SiF₄ concentration as a function of time, with Si present,are shown in FIG. 28.

FIG. 29 shows the difference in voltage drop between the baselineexperiment and the experiment with silicon. The increase in voltage dropcorrelated extremely well with the increase in SiF₄ generated, and thereaction appeared to be reversible, with the Pt resistance returning tothe pre-exposure value. The resistance increase in the Pt foil was muchgreater than that calculated based on the known temperature coefficientof resistivity; consequently, this increase in resistance was attributedto a chemical interaction between the NF₃ plasma, Si effluents and thePt foil.

EXAMPLE 2

To demonstrate the use of three-dimensional architectures, threeprototype sensors were constructed of machined Vespel® polyimide asshown in FIGS. 30-35.

FIGS. 30 and 31 show respectively a side view (FIG. 30) and a top view(FIG. 31) of a machined Vespel® polyimide sensor array utilizing acylindrical pegboard design mounted on an aluminum flange, wherein eachsensing wire has four-point contact via press-fitted posts.

FIGS. 32 and 33 show respectively a side view (FIG. 32) and a top view(FIG. 33) of a machined Vespel® polyimide sensor assembly utilizing anorthogonally-cut Vespel® polyimide post and channels design (withshelves) mounted on an aluminum flange, wherein each sensing wire hasfour-point electrical contact via press-fitted posts.

FIGS. 34 and 35 show respectively a side view (FIG. 34) and a top view(FIG. 35) of a machined Vespel® polyimide sensor assembly mounted on analuminum flange, utilizing Vespel® polyimide cylinders with spiralinggrooves of varying lengths allowing a single sensing wire to be woven“up” and “down” the length of a single Vespel® polyimide cylinder.Multiple cylinders allow the use of multiple sensing wires, wherein eachsensing wire has four-point electrical contact via press-fitted posts.

The sensor of FIGS. 30-31 was machined from a Vespel® polyimide cylinderto replicate the non-planar pegboard structure hereinabove in FIG. 25.This prototype was contructed in a multiple sensing array, utilizingplatinum, nickel, copper and aluminum wires in a 16-pin, four-pointelectrical contact per wire format.

The sensor shown in FIGS. 32-33 was machined to replicate theorthogonally-cut Vespel® polyimide structure with channels, posts andshelves illustrated in FIG. 20. This prototype was constructed in amultiple-sensing array utilizing copper and aluminum wires in an 8-pin,four-point electrical contact per wire format.

The sensor of FIGS. 34-35 represents a variation on the channels andshelves design. The channels in this prototype were cut in a spiralpattern along the length of a single cylindrical Vespel® polyimide post,and the sensing wire was woven inside the channel. By cutting twoseparate spiral channels of different depths, which criss-cross oneanother along the length of the Vespel® polyimide post, the sensing wirecan be made to make two passes along the length of the post, being wovenup the post in one groove and down the post in a second groove. Becausethe posts have different depths, the wire does not contact itself as itpasses itself in the groove intersections. This prototype wasconstructed in a multiple sensing metal array, utilizing platinum,nickel, copper and aluminum wires in a 16-pin, four-point electricalcontact per wire format.

EXAMPLE 3

In this example, data was generated in the course of a 100 wafer run. Agas sensor assembly was constructed using a cylindrical pegboard memberproviding a scaffolding support structure for a nickel sensing wire anda copper sensing wire. The copper wire was positioned at an upperportion of the cylinder, and the nickel sensing wire was positioned at alower portion of the cylinder. A 75 milliamp constant DC current waspassed through the nickel wire and the copper wire in series.

Each of the nickel and copper wires was nominally 100 microns indiameter and approximately 13-14 centimeters in length. The electricalresistance of the nickel wire was ˜1.3 ohms, and the resistance of thecopper wire was ˜0.255 ohms.

Gaseous effluent from a process chamber undergoing successive depositionand chamber clean operations was contacted with the sensing wires andthe rate of change of the resistivity of the gas sensing wires wasmonitored as a function of time during both the deposition step and thechamber clean step.

The deposition step involved deposition of silicon on a substrate from atetraethylorthosilicate (TEOS) source reagent, followed by the cleaningof the chamber with NF₃. In order to correlate the dynamic resistivitybehavior of the gas sensing wires with the process operation, effluentgas from the chamber after contacting with the gas sensing assembly waspassed through a residual gas analyzer (RGA) unit. The output of the RGAunit was monitored as a function of time, and the graphical outputs ofthe gas sensing assembly and the RGA unit were superposed as a functionof time, as shown in the graph of FIG. 36.

The process conditions that were employed in the constituent depositionand clean steps are set out below.

TEOS Deposition

-   Chamber Pressure=9 Torr-   Chamber Temp=390° C.-   RF Power=350 Watts-   TEOS Flow Rate=230 sccm-   Helium Flow Rate=100 sccm-   Oxygen Flow Rate=220 sccm

Each TEOS deposition was 120 seconds in duration.

NF₃ Clean Operation

-   Chamber Pressure=3.2 Torr-   Chamber Temperature=390° C.-   RF Power=350 Watts-   Helium Flow Rate=225 sccm-   NF₃ Flow Rate=100 sccm

Each NF₃ clean cycle was 200 seconds in duration.

FIG. 36 is a graph of the behavior of the gas sensing assembly as an endpoint monitor (EPM), shown in the upper portion of the graph, andresidual gas analyzer (RGA) gas concentration, shown in the lowerportion of the graph, as a function of time, in minutes.

Curves A and B show the change in resistance, in units of(ohms/minute)×10⁻³, as a function of time, for successivetetraethylorthosilicate (TEOS) deposition steps and alternating nitrogentrifluoride (NF₃) cleaning steps in the previously described processoperation.

The lower portion of FIG. 36 shows the residual gas analyzer monitoredconcentrations of fluorine (Curve C) and silicon tetrafluoride (Curve D)as a function of time, in minutes.

It is apparent from the superposition of the upper and lower portions ofthe graph in FIG. 36 that the resistance change of the end point monitorsensing element (wire) in the test correlated well with the fluorospecies detection by the residual gas analyzer, showing that the gassensing assembly provided a highly effective means of monitoring fluorospecies in the chamber clean operations.

Although the invention has been variously described herein withreference to illustrative embodiments and features, it will beappreciated that the embodiments and features described hereinabove arenot intended to limit the invention, and that other variations,modifications and other embodiments will readily suggest themselves tothose of ordinary skill in the art, based on the disclosure herein. Theinvention therefore is to be broadly construed, consistent with theclaims hereafter set forth.

1. A gas sensor assembly comprising a micro-hotplate structure includinga free-standing gas sensing element responsive to presence of fluorospecies by response indicative of presence or increase in concentrationof said fluoro species.
 2. The gas sensing assembly of claim 1, whereinthe gas sensing element comprises a metal selected from the groupconsisting of Ti, V, Cr, Mn, Nb, Mo, Ru, Pd, Ag, Ir, Ni, Al, Cu, Pt, andalloys and combinations thereof.
 3. The gas sensing assembly of claim 1,wherein the gas sensing element comprises an inorganic sensor film. 4.The gas sensing assembly of claim 1, wherein the gas sensing elementcomprises an organic sensor film.
 5. The gas sensing assembly of claim3, wherein said gas sensing element comprises a metal selected from thegroup consisting of copper and nickel.
 6. A gas sensor assemblycomprising a free-standing gas sensing element coupled to connector pinsof a microelectronic device package, wherein the free-standing gassensing element is arranged for contact with a gaseous environmentsusceptible to the presence or change of concentration of one or moretarget gas species therein, and the free-standing gas sensing element isformed of a material that in exposure to the target gas species exhibitsa response transmissible through said connector pins of themicroelectronic device package.
 7. The gas sensor assembly of claim 6,wherein the microelectronic device package comprises a chip carrier. 8.The gas sensor assembly of claim 6, wherein the free-standing gassensing element comprises a wire or foil element.
 9. The gas sensorassembly of claim 6, wherein the free-standing gas sensing elementcomprises a wire supported on an insulative scaffolding member in anextended conformation including a multiplicity of windings.
 10. The gassensor assembly of claim 9, wherein the insulative scaffolding member isformed of Vespel® polyimide polyimide.
 11. The gas sensor assembly ofclaim 6, as coupled in target gas-sensing relationship to asemiconductor process chamber.